Lime sulfur, which is permitted by the Ontario Pesticides Ban, can also be used as a dormant spray to control peach leaf curl, applied in early spring before bud break.

Unfortunately, no other natural controls are available for an existing tree. Choosing resistant cultivars is recommended. Also, as the spores are washed into the buds by spring rains, one recommendation is to espalier the tree in a sheltered spot, such as beneath a roof overhang. Neither of these might be possible for you, but file the info away for future reference.